Golf balls having various structures have been devised, and include one-piece golf balls consisting of a golf ball body, two-piece golf balls including a core and a cover, three-piece golf balls including a core composed of a center and a single intermediate layer covering the center, and a cover covering the core, and multi-piece golf balls including a core composed of a center and at least two intermediate layers covering the center, and a cover covering the core.
Ionomer resins are widely used as materials for golf balls because these ionomer resins enable golf balls to be very rigid and achieve a long flight distance. In particular, highly resilient materials are demanded. Ionomer resins having an increased degree of neutralization are known to increase resilience. Unfortunately, such highly neutralized ionomer resins have low fluidity and thus are difficult to mold. Further, as the degree of neutralization is increased, resilience is improved, but hardness tends to undesirably increase at the same time, which results in lower flexibility and poorer shot feel.
To solve this problem, a method for reducing hardness while increasing resilience by adding a large amount of a fatty acid (metal soap) to a highly neutralized ionomer resin has been proposed. Unfortunately, the acid component in the fatty acid consumes metal ions used for neutralization, and prevents the highly neutralized ionomer resin from providing the effect of increasing resilience sufficiently. Thus, the method has an insufficient effect in increasing flexibility and resilience in order to provide a golf ball that simultaneously achieves good shot feel and resilience. Moreover, the method needs a large amount of the metal component.
Further, Patent Literature 1 discloses a golf ball material prepared using an ionomer resin and a compound containing both acid and amine functional groups in the same molecule. Patent Literature 2 discloses a golf ball formed using an acid polymer containing 70% or more-neutralized acid groups and a polyhydric alcohol. Patent Literatures 3 and 4 disclose resin compositions for golf balls including an ionomer resin and a compound containing two or more reactive functional groups and having a molecular weight of 20,000 or less. Moreover, Patent Literature 5 discloses a resin composition for golf balls including a thermoplastic resin such as a terionomer, and teaches that a fatty acid derivative such as an alcohol fatty acid ester, glycerol fatty acid ester, or glycol fatty acid ester may be added in the resin composition.
Unfortunately, the golf ball materials disclosed in these literatures still leave something to be desired in terms of increasing flexibility and resilience at the same time. They are also demanded to simultaneously have good fluidity.